1. Field of the Invention
This invention relates to an apparatus for controlling dampening water circulatively used in offset printers. More particularly, it is concerned with an improvement in a dampening water controller or controlling apparatus for controlling the concentration of etching solution in dampening water containing an alcohol or surface active agent and an etching solution as well as impurities such as ink and paper dust that can be brought into the dampening water as it is used circulatively.
2. Description of the Prior Art
In offset printing, what is called dampening water is usually made to adhere to non-image areas on which no ink must be laid, to cause that areas to repel ink so that the ink can be controlled to adhere or not to adhere according to a pattern on a plate. Accordingly, how the dampening water is maintained and controlled to function at a high performance is a very important subject for the quality of prints obtained by offset printing. Now, the dampening water usually contains water, an etching solution (a solution added to dampening water when used) and an alcohol such as isopropyl alcohol or ethyl alcohol or a surface active agent.
The alcohol or surface active agent, though contained in a trace amount, decreases the surface tension of the dampening water to thereby improve its wettability to a plate and so functions as to uniformly spread the dampening water over the whole plate. That is, the alcohol or surface active agent contained in an appropriate amount greatly contributes a fundamental performance of the offset printing.
The etching solution also contains gum arabic, which is a colloidal substance comprised of a water-soluble polymer. The gum arabic has what is called a desensitizing action. This desensitizing action is greatly affected by pH values, and is considered to exhibit its highest function in a certain specific pH range. For this reason, anions such as nitrate ions, nitrite ions, phosphate ions, fluoride ions, chloride ions, sulfate ions or sulfide ions or cations such as sodium ions, ammonium ions, calcium ions or potassium ions are usually mixed in the etching solution. Using these various kinds of ions and a pH adjuster optionally added, the pH value of the dampening water containing the etching solution is adjusted so as to be set in a specific pH range, usually in a pH range of from 4.0 to 6.5.
As apparatus for controlling this dampening water, a system has been hitherto employed in which the pH and alcohol concentration of dampening water being used are monitored and the concentration of etching solution and concentration of alcohol in the dampening water are controlled in accordance with the resulting data.
Incidentally, when the dampening water is circulatively used (which is commonly circulated for the purpose of, e.g., saving the dampening water.), impurities such as ink and paper dust are usually brought into the dampening water that has returned without being consumed on a printer. Now, the paper commonly contains calcium carbonate as its component, and hence the pH value of the dampening water tends to become higher with an increase in the quantity of calcium carbonate in the dampening water. For this reason, the control made in accordance with pH values of the dampening water has been disadvantageous in that the presence of calcium carbonate makes it difficult to accurately judge the concentration of etching solution in the dampening water. Namely, what is important as characteristic values for controlling the dampening water is not the pH values but the etching solution concentration itself in the dampening water.
Under such circumstances, as disclosed, for example, in Japanese Patent Application Laid-open No. 56-157360 or No. 63-1543, it is attempted to monitor conductivity of dampening water being used and control the concentration of etching solution in accordance with the resulting data. This method, though it is based on not the controlling of the etching solution concentration itself but the idea that the conductivity is proportional to the concentration of etching solution, has an advantage that the measurement itself is very easy.
However, according to this method, a conductivity of the dampening water having returned from a water fountain (a long and slender pad containing dampening water, which is provided in the circulation path of dampening water and through which the dampening water is fed to the surface of a plate) is measured. Hence, when the above impurities have been brought into the dampening water as a result of its circulative use, the impurities may affect the dampening water to change its conductivity, resulting in a break of the relationship between the concentration of etching solution in the dampening water and the conductivity of the dampening water. Thus, there has been the problem that the concentration of etching solution in the dampening water can be misjudged.
Here, the concentration (%) of etching solution in the dampening water is defined as: EQU 100Y/(X+Y+Z) (%)
wherein X represents a quantity of water before the mixing of a stock solution of the etching solution, Y represents a quantity of a stock solution of the etching solution, and Z represents a quantity of an alcohol.
With regard to the concentration (%) of alcohol in the dampening water, it is defined as follows: EQU 100Z/(X+Y+Z) (%)